feat(ui): login + conflict dialogs (Qt)

Login dialog (prompt_login_qt):
  Modal asking for a discord_id:password token. Echo masked. Validates
  format before accepting. 'Skip cloud sync' returns None so caller can
  bypass sync without blocking the launch.

Conflict dialog (prompt_conflict_qt):
  Modal with three radio options — keep_local, use_remote, cancel.
  Shown when local files were modified since the last pulled snapshot
  AND the remote has a newer snapshot from another host (divergence
  state.json detection happens in sync.pull; not wired yet).

Both reuse the Prism dark palette via _apply_prism_dark. Tk fallbacks
not implemented — Qt is the path most players will hit.

Next: state.json (last_pulled_snapshot_id), divergence detection in
sync.pull, integration with these dialogs.

cloud_sync/ui_qt.py

@@ -184,3 +184,171 @@ class QtProgressWindow:
 
     def _on_finished(self, _rc: int) -> None:
         self._dialog.accept()
+
+
+# ---------------------------------------------------------------------------
+# Login dialog (first-run token capture)
+# ---------------------------------------------------------------------------
+
+
+def prompt_login_qt() -> str | None:
+    """Modal dialog to collect a fresh ``discord_id:password`` token.
+
+    Returned string is the validated raw token (caller writes to disk).
+    Returns ``None`` if the user picked "Skip cloud sync" or closed the
+    dialog — caller should treat that as "don't sync, don't block launch."
+    """
+    QtWidgets, QtCore, _ = import_qt()
+    app_existed = QtWidgets.QApplication.instance() is not None
+    app = QtWidgets.QApplication.instance() or QtWidgets.QApplication(sys.argv)
+    if not app_existed:
+        _apply_prism_dark(app)
+
+    dialog = QtWidgets.QDialog()
+    dialog.setWindowTitle("Cloud sync — first time setup")
+    dialog.setFixedSize(480, 260)
+
+    layout = QtWidgets.QVBoxLayout(dialog)
+    layout.setContentsMargins(20, 20, 20, 20)
+    layout.setSpacing(10)
+
+    title = QtWidgets.QLabel("Connect to cloud save")
+    font = title.font()
+    font.setBold(True)
+    font.setPointSize(font.pointSize() + 2)
+    title.setFont(font)
+    layout.addWidget(title)
+
+    body = QtWidgets.QLabel(
+        "In Discord, message the bot:\n"
+        "    /cloud register\n\n"
+        "It will DM you a one-line token. Paste it below:"
+    )
+    body.setWordWrap(True)
+    layout.addWidget(body)
+
+    field = QtWidgets.QLineEdit()
+    field.setPlaceholderText("123456789012345678:a1b2c3d4…")
+    field.setEchoMode(QtWidgets.QLineEdit.EchoMode.Password)
+    layout.addWidget(field)
+
+    error = QtWidgets.QLabel("")
+    error.setStyleSheet("color: #ff6b6b;")
+    layout.addWidget(error)
+
+    layout.addStretch(1)
+
+    button_row = QtWidgets.QHBoxLayout()
+    skip = QtWidgets.QPushButton("Skip cloud sync")
+    button_row.addWidget(skip)
+    button_row.addStretch(1)
+    save = QtWidgets.QPushButton("Save and continue")
+    save.setDefault(True)
+    button_row.addWidget(save)
+    layout.addLayout(button_row)
+
+    chosen: dict[str, str | None] = {"value": None}
+
+    def on_save() -> None:
+        token = field.text().strip()
+        if ":" not in token:
+            error.setText("Token must be discord_id:password")
+            return
+        head, sep, tail = token.partition(":")
+        if not head.isdigit() or not tail:
+            error.setText("discord_id must be numeric; password must be non-empty")
+            return
+        chosen["value"] = token
+        dialog.accept()
+
+    def on_skip() -> None:
+        chosen["value"] = None
+        dialog.reject()
+
+    save.clicked.connect(on_save)
+    skip.clicked.connect(on_skip)
+    field.returnPressed.connect(on_save)
+
+    dialog.exec()
+    return chosen["value"]
+
+
+# ---------------------------------------------------------------------------
+# Conflict dialog (local-modified-since-last-sync + remote-newer)
+# ---------------------------------------------------------------------------
+
+
+def prompt_conflict_qt(
+    local_summary: str,
+    remote_summary: str,
+) -> str:
+    """Ask how to resolve a divergence.
+
+    Returns one of: ``"keep_local"``, ``"use_remote"``, ``"cancel"``.
+    """
+    QtWidgets, QtCore, _ = import_qt()
+    app_existed = QtWidgets.QApplication.instance() is not None
+    app = QtWidgets.QApplication.instance() or QtWidgets.QApplication(sys.argv)
+    if not app_existed:
+        _apply_prism_dark(app)
+
+    dialog = QtWidgets.QDialog()
+    dialog.setWindowTitle("Cloud sync — conflict")
+    dialog.setFixedSize(520, 320)
+
+    layout = QtWidgets.QVBoxLayout(dialog)
+    layout.setContentsMargins(20, 20, 20, 20)
+    layout.setSpacing(10)
+
+    title = QtWidgets.QLabel("Local and cloud diverged")
+    font = title.font()
+    font.setBold(True)
+    font.setPointSize(font.pointSize() + 2)
+    title.setFont(font)
+    layout.addWidget(title)
+
+    summary = QtWidgets.QLabel(
+        f"Local changes since last sync:\n  {local_summary}\n\n"
+        f"Cloud has newer changes from another machine:\n  {remote_summary}\n\n"
+        f"What should we do?"
+    )
+    summary.setWordWrap(True)
+    layout.addWidget(summary)
+
+    keep_local = QtWidgets.QRadioButton(
+        "Keep local — skip pull. Next exit will overwrite cloud."
+    )
+    use_remote = QtWidgets.QRadioButton(
+        "Use cloud — overwrite local files with the cloud version."
+    )
+    cancel_launch = QtWidgets.QRadioButton(
+        "Cancel launch — don't start Minecraft until I resolve manually."
+    )
+    keep_local.setChecked(True)
+    layout.addWidget(keep_local)
+    layout.addWidget(use_remote)
+    layout.addWidget(cancel_launch)
+
+    layout.addStretch(1)
+
+    button_row = QtWidgets.QHBoxLayout()
+    button_row.addStretch(1)
+    confirm = QtWidgets.QPushButton("Continue")
+    confirm.setDefault(True)
+    button_row.addWidget(confirm)
+    layout.addLayout(button_row)
+
+    chosen: dict[str, str] = {"value": "cancel"}
+
+    def on_confirm() -> None:
+        if keep_local.isChecked():
+            chosen["value"] = "keep_local"
+        elif use_remote.isChecked():
+            chosen["value"] = "use_remote"
+        else:
+            chosen["value"] = "cancel"
+        dialog.accept()
+
+    confirm.clicked.connect(on_confirm)
+    dialog.exec()
+    return chosen["value"]